LIMESTONES AND MARLS
1. Partial Analysis
This is limited to estimation of the calcium carbonate and any clay
The proportion of calcium carbonate may be deduced with sufficient
exactness (when only little magnesia is present) by a gasometric determina-
tion of the carbon dioxide (see later, Complete Analysis, 4) or by the following
i gram of the powdered material is boiled in a flask with a little water
and 25 c.c. of N-hydrochloric acid to expel the carbon dioxide, the excess
of acid in the cold liquid being titrated with N-caustic soda in presence
of cochineal. The number of c.c. of acid neutralised, multiplied by 5, gives
the percentage of calcium carbonate.
If this determination indicates that the proportion of clay present is
not negligible, approximate estimation is made of the clay (regarding as
such the silica, plus the alumina and ferric oxide). For this purpose 2
grams of substance are treated in a fairly large porcelain dish with about
150 c.c. of water and 10 c.c. of concentrated hydrochloric acid (added care-
fully)/the liquid being boiled for some minutes and the alumina and ferric
oxide precipitated, without preliminary nitration, by slight excess of ammo-
nia. The insoluble matter (silica) and the precipitate are then collected
on a filter, washed 5 or 6 times with water, dried, ignited at a dull red heat
in a crucible and weighed.
2. Complete Analysis
For a detailed analysis of a limestone or marl the following determina-
tions are to be made :
1. Moisture (hygroscopic water).—5-10 grams of the finely powdered
substance are dried in an oven at 105-110° until constant in weight.
2. Loss on Ignition (combined water + carbon dioxide + organic
matter).—1-2 grams of the dry substance are heated in a platinum crucible,
at first gently over a bunsen flame and later for half an hour over a blow-
pipe flame, this ignition being repeated until no further loss of weight occurs.
3. Combined Water.—1-2 grams of the dry substance are heated in
a boat in a hard glass tube traversed by a current of dry air, the issuing
gas being passed through a calcium chloride tube. The increase in weight
of the latter gives the combined water, while the loss in weight of the boat
represents the loss on ignition. This determination renders that given
under 2 (above) unnecessary.
When organic matter is present, the result of this determination is not very
exact, the combined water being increased by that formed by the combustion
of the hydrogen of the organic matter.
4. Carbonic Anhydride.—This may be estimated gravimetrically or
by measuring the gas evolved when the substance is treated with hydro-
(a) GRAVIMETRIC METHODS. These are based on the loss in weight ofis attaches to the